What happened?

The Fermi-1 SFR had encountered some abnormal temperature measurements and suspected a flow blockage. To investigate, they moved some assemblies around and came to power. On October 5, 1966, at about 30% full power, radiation alarms went off and the reactor was shut down.

Two assemblies had partially melted because the coolant flow had been 97% obstructed from below. It was determined that a zirconium segment (installed in a conical flow guide/core catcher in 1959 to mitigate vessel break/complete loss of coolant) had broken off and caused the blockage.

Lessons

1. Design your inlets to be difficult to block

2. Don’t put stuff that can break off and block things in your core

3. Be able to detect flow blockage early with instrumentation and analysis

A computer system (Malfunction detection analyzer) was planned to be installed to evaluate reactivity every 5 seconds and alert operators of any deviations (as small as 2 cents) from expectations in power, flow rate, and inlet temperature

Legacy

Most inlet nozzles since Fermi-1 have featured bayonet-style inlets with holes on the sides so as to make it much more difficult geometrically for a loose part to significantly block flow

References and additional reading

1. REPORT ON THE FUEL MELTING INCIDENT IN THE ENRICO FERMI ATOMIC POWER PLANT ON OCTOBER 5, 1966. p., 1968. Web. doi:10.2172/4766757. 2